UV maping and textures for low poly mesh
Yeah. Basically you need to bake the hi-poly model's texture upon the low-poly model.
So: 


retopo your hipoly model to get the lowpoly. 
UVmap the lowpoly one and create a new image upon wich you'll bake the hipoly texture. Make sure this is the image asigned to the lowpoly model when you go into edit mode. If it's not, just select it from the dropping menu, it should stay.
with both models in the same location (overlapped) and selected (being the lowpoly the active one), go to properties bar > render > bake. Choose TEXTURE in the bake mode menu, and make sure SELECTED TO ACTIVE is checked. I usually uncheck CLEAR too. And press BAKE.
this way the texture from the hipoly model should be baked upon your new image, using the lowpoly model's UVmap. 


hopefully thatll help. Im a noob myself so take that with some reserve tho.